WASHINGTON - Vice President Joseph Biden says the White House remains open to direct talks with Iran over its nuclear enrichment program, but the U.S. would refuse to engage Tehran until its government and Ayatollah Ali Khamenei demonstrated they were "serious" about moving forward.

Speaking at a security conference in Munich, Germany, today the vice president said the administration had been clear it "would be prepared" to meet for one-on-one negotiations between the states, outside the multilateral meetings that have not convened for major talks since June.

"We would not make it a secret that we were doing that," Biden said. "We would let our partners know if that occasion presented itself. That offer stands, but it must be real and tangible, and there has to be an agenda that they're prepared to speak to. We are not just prepared to do it for the exercise."

Any thawing of the chill between Tehran and Washington suffered another setback last month when Iran announced it would accelerate enrichment of uranium that could be used in fuel reactors as well as nuclear warheads. The decision came ahead of six-power talks expected to resume soon, although the State Department said Iranian leadership has continually changed their preconditions and dates.

Also this week, Iran claimed to have launched a rocket carrying a monkey into space. While analysts have cast serious doubt over the veracity of the claim, it dovetailed with continued international concern the country's "space program" was being used as a testing bed for ballistic missile technology.

Biden said it wasn't too late to work within the confines of the multilateral discussions with the five permanent members of the U.N. Security Council, plus Germany.

"There is still time, there is still space for diplomacy, backed by pressure, to succeed," he told the conference. "The ball is in the government of Iran's court, and it's well past time for Tehran to adopt a serious, good-faith approach to negotiations with the P5-plus-1."

In December, Iranian foreign minister Ali Akbar Salehi told state-run media that his government was also open to bilateral discussions, but that approval would ultimately come from the ayatollah. The minister is at the Munich convention and is expected to make remarks Sunday, although there is no meeting scheduled between the two leaders.

This position on bilateral talks is not a new stance for the Obama administration. But last October the New York Times reported the White House had taken it a step further, with the U.S. and Iran agreeing, in principle, to one-on-one discussions. Citing anonymous members of the Obama administration, the paper said Iran had insisted on delaying negotiation until after the U.S. presidential election in November.

At the time, the White House denied the Times' report that any such agreement had been met.

While the six-power discussions have gained little traction since the summer, international trade sanctions on Tehran continue to cut into its oil and financial infrastructure.